Removing a Dark Ring

Tips on getting dark stains out of wood with oxalic acid and bleaching agents. July 13, 2005

Question
I have a Mahogany tabletop with a dark ring stain. The dark stain outlines the ring, and is also inside the ring. Does anyone have any suggestions on how to fix this? I would love to keep the color of the top, but I feel as though bleaching may be the only way to keep the color.

Forum Responses
(Finishing Forum)
From contributor T:
Sounds like you have it stripped. The first thing to try is a saturated solution of oxalic acid. It won't bleach the wood appreciably, but it might lighten any residual stain, so go over the entire top. Try neutralizing it with diluted vinegar and rinsing it thoroughly. Oxalic acid is a very weak acid and won�t burn skin or clothing, but the crystals or sanding dust is something you don't want to inhale.

From contributor C:
Contributor T, I�m not clear on why you would use vinegar to neutralize Oxalic acid - as they are both acids (vinegar is acetic acid in very dilute solution). Normally an acid is neutralized with a base solution such as baking soda.

Personally, I have found it difficult to neutralize such treatments in a reliable way, and I rely on thorough washing instead. I do agree that Oxalic acid is exceptionally effective on iron/tannin stains, which are the main sources of black stains on furniture. The amazing thing is how the Oxalic acid specifically attacks the iron stains while having very little effect on the wood colors. I have had little success in totally removing such stains, but in most cases I have been able to lighten them quite a lot, which makes considerable improvements.

From contributor R:
I�ve used Oxalic acid to remove water marks as well as ink stains. All that has to be done to the crystals once they have solidified is to get a good washing with very hot water. I would think that ammonia or vinegar might possibly cause a problem down the road.

One of the best bleaches I�ve used has been the Wood-Kotes brand. It�s a bleach for lightening woods - not removing mineral marks or stains, and it states that no neutralizing is necessary.

From contributor J:
Contributor R, Speed-way was the best bleach I've used. As far as neutralizing fuming or acid treatments, I believe that it is a must. Since it has reached its maximum effect on the wood, it has burnt itself out of its acidic nature.